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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

SILVER ALLOYS

•281

dilute sulphuric acid, dried, re-melted alone in a smaller crucible and poured
into a small ingot-mould heated and greased with tallow or vaseline. The
ingot obtained is thoroughly cleaned, washed first with dilute sulphuric
acid and then with water and dried. It is then cut into pieces with a chisel,
and these rolled into strips easily cut with the metal shears.1

Titration of the thiocyanate solution. Exactly 0-2 gram of fine silver is
heated gently with 5-10 c.c. of nitric acid (D 1-2) in a conical flask covered
with a funnel until the metal is dissolved and the red fumes have disappeared.
The funnel is removed after cooling and washed with distilled water, the
solution being treated with 50 c.c. of cold water and 2-3 c.c. of the ferric
alum solution, and the thiocyanate solution gradually run in from a burette
until the milky liquid assumes a persistent pink tint. It is usual to adjust
the strength of the thiocyanate solution so that 0-2 gram of silver requires
exactly 50 c.c.

Procedure in the actual test. Exactly 0-2 gram of the sample is dissolved
in nitric acid 2 as described above and the solution diluted and titrated
with the thiocyanate.                                                                            ,

The Volhard method is very rapid and applicable to alloys of any degree
of fineness but is not so exact as Gay-Lussac's method (c). Further, it cannot
be used in presence of mercury or palladium, since these metals also react with
thiocyanate. Also, with more than 70 % of copper, the blue coloration renders
the end-point less exact; in this case fine silver may be added so as to diminish
the proportion of copper. Nickel and cobalt have a similar effect to copper.

(c) GAY-LUSSAC'S METHOD. This was proposed in 1832, when the
French Minister of Finance appointed a commission, of which Gay-Lussac
was a member and also reporter, to study the causes of error in the deter-
mination of silver by the cupellation method.8

It consists hi adding to a nitric acid solution of the sample sufficient
sodium chloride solution to precipitate almost the whole of the silver, and
in estimating the small amount of silver remaining in solution from the
faint turbidity produced by addition of a fresh quantity of the sodium
chloride solution. With a little practice, I part in 10,000 may be deter-
mined accurately by this method. Since it requires a knowledge of the
approximate composition of the sample, a preliminary test by Volhardjs
method or by cupellation is necessary.

Apparatus, i. Ordinary bottles of about 200 c.c. capacity, fitted with
tight-fitting ground stoppers and with a distinctive mark on both bottle
and stopper.

2. A^ioo c.c. pipette. The pipettes used by assayers are usually without
a stem and, to facilitate reading, are fixed in a stand (Fig. 24).

Increased accuracy of measurement is, however, obtained by means of
the Stas pipette (Fig. 25), which is a 100 c.c. pipette drawn out to a point

1  This method for obtaining fine silver is that adopted by the testing laboratory
of the Royal Italian Mint at Rome.

2  No nitrous fumes should be present in the nitric acid used and those formed
during the reaction should be completely expelled, since nitrous fumes—and nitric
acid itself in the hot—decompose thiocyanates.

3  Gay-Lussac : Instruction sur I'essai des matteres a'argent par la voie humide, Paris,
Imprimerie royale.n and the metallic button washed with